1. Field of the Invention
The invention relates generally to the field of oil and gas well logging. More specifically, the present invention relates to a method and apparatus to enhance the accuracy of retrieving acoustical data gathered during sub-sea oil and gas well logging.
2. Description of Related Art
Numerous techniques, generally known as well logging, exist for collecting geological data from oil and/or gas wells, where the geological data is useful for locating potential hydrocarbon bearing reservoirs. Well logging is also used for estimating the capacity of the potential hydrocarbon bearing reservoirs. Many types of well logging practices exist. They include neutron logs, induction logs, and acoustic logs. In each of the aforementioned well logging techniques a well logging tool is deposited into the wellbore and travels through the well bore collecting geological data about the region surrounding the well bore. Generally the well logging tool produces a signal, either electrical, nuclear, or acoustical, which is directed into the area adjacent the well bore. The reflection or propagation of the emitted signal is then retrieved by the tool or by another piece of equipment suitably located. The retrieved signals are stored and analyzed in order to evaluate the potential for hydrocarbon production in the particular geological formation being analyzed, monitor reservoir performance, or to evaluate wellbore mechanical integrity.
Generally the well logging tool is inserted into the well bore attached to a wire line. The tool is raised and lowered by the wireline, and data is transmitted through the wireline for introducing signals to the well logging tool from the surface. The wire line can also transmit data recovered from within the well bore to the surface for collection and analysis. Because in most instances the bore hole pressure exceeds the ambient pressure, weighted wellbore fluid is used to overbalance or contain this pressure. As a secondary safety measure, a pressure containment apparatus, or pack off head, is often installed on the well during wireline operations. A typical pack off head includes a hard rubber insert with a passage where the wire line passes through the annulus. To seal around the wireline, the hard rubber insert is axially compressed, which reduces the cross sectional area of the passage. Reducing the cross sectional area of the passage causes the inner radius of the passage to fit snugly around the outer radius of the wire line, thus preventing fluid flow through the passage. Although the passage snugly seals around the outer radius of the wire line, the wire line is still able to freely traverse through the passage.
In acoustic well logging, acoustic waves are emitted from a transmitter source and travel through the casing, cement sheath, and geological formations that surround the well. Receivers are situated at predetermined locations and distances away from the acoustic source. The receivers are able to detect the waves and then measure the wave frequencies and velocities. Measurement of the wave frequencies and velocities can provide useful information regarding the potential for hydrocarbon production, reservoir performance, or wellbore mechanics.
As is well known in the art, additional well logging activities occur after an oil or gas well has been completed. Completing an oil or gas well involves inserting metal casing into an already drilled well bore. After the casing is inserted into the larger wellbore, an annular space is formed between the outer surface of the casing and the inner radius of the wellbore. Cement is then injected into this space thereby securing the casing into the wellbore and generally providing a sealing bond between the casing and the inner radius of the wellbore.
The casing extends into the well bore often in excess of 5000 feet, sometimes greater than 15,000 feet. Consequently the formation pressure of the fluid (either hydrocarbon or water) in the surrounding geological formation can be quite high. A drilling fluid or mud is maintained in the wellbore during drilling and cementing procedures at a density to provide an overbalanced condition to contain the formation pressures anticipated in the wellbore. After the cementing procedure is completed, and the formations are sealed off from the wellbore, heavy drilling mud is replaced in the casing in preparation for the final well completion of the well for production. The heavy drilling mud is replaced with a lighter weight fluid (completion fluid) that is pumped into the wellbore. When the heavy drilling mud inside of the casing is replaced with the lower specific gravity completion fluids, a pressure differential will result across the diameter of the casing. With reduced pressure on the inside of the casing, compared to the higher pressure on the outer surface of the casing, the casing will contract leaving a small void, termed a micro-annulus, between the outer surface of the casing and the cement that surrounds the casing. The presence of a micro-annulus alters the acoustic signal path which in turn affects the acoustic cement evaluation instruments that are used to measure the bonding condition of the cement to the pipe and to the formation.
It is possible to eliminate the micro-annulus affect if the inside of the casing is sufficiently pressurized to expand the casing up against the inside of the cement sheath simulating the drilling mud during cementing operations, while the cement bond log is performed. While this can currently be performed above ground, this is not the case with deepwater offshore or subsea wells. The current deepwater sea floor well head and riser configuration prevents the operators from pressurizing the inner casing without applying unacceptable pressure on the riser.
Therefore, a method or an apparatus is desired that enables pressurizing the inside of an offshore or subsea oil and/or gas well casing with sufficient pressure to eliminate any micro-annulus voids that may exist between the casing and the cement sheath, without resulting in an unacceptable pressure being exposed to the drilling riser.
A seafloor pressure head assembly for use in hydrocarbon producing wellbore wireline operations in combination with a blow out preventer, where the blow out preventer is located at the entrance to a subsea hydrocarbon producing wellbore. Formed within an inner axial passage of the blow out preventer is an inflatable bladder. Also attached to the blow out preventer is a hollow riser that reaches to, or proximate to, the sea surface.
The seafloor pressure head assembly is comprised of an elongated tube, a pack off head, a stop ring, centralizers, and a re-entry skirt. The elongated tube is typically cylindrical and hollow. Coaxially connected to the top of the elongated tube is the pressure pack off head formed to receive a wireline therethrough. A pressure seal is provided in the pack off head where the wireline enters the pack off head that prevents pressure communication between the inside of the pack off head and the ambient space surrounding the pack off head. The elongated tube bottom is formed to be inserted into the blow out preventer in a way that the inflatable bladder of the blow out preventer circumferentially surrounds the elongated tube along a discrete axial distance. The inflatable bladder surrounds the elongated tube between the wellbore pressure side and the ambient pressure side. When the elongated tube is inserted into the blow out preventer an annulus is formed between the outside of the elongated tube and the inner radius of the blow out preventer.
When the inflatable bladder is inflated it produces an inflated bladder. The inflated bladder circumferentially engages the elongated tube along the discrete axial distance and occupies the portion of the annulus between the elongated tube and the inflatable bladder. Engaging the elongated tube provides a restraining force of sufficient magnitude to attach the elongated tube to the blow out preventer. The inflated bladder also provides a pressure seal in the portion of the annulus between the elongated tube and the inflatable bladder.
Also included in the sea floor pressure head assembly is a stop ring circumferentially attached to the elongated tube high pressure side. The stop ring, which is securedly affixed to the elongated tube, is prevented from traversing across the blow out preventer by contacting the lower edge of the inflated bladder.
A method is provided for eliminating the presence of microannuluses associated with a subsea hydrocarbon producing wellbore for accurate gathering of data within the subsea hydrocarbon producing wellbore. The method involves using a sea floor pressure head assembly in conjunction with a subsea blow out preventer, where the sea floor pressure head assembly includes an elongated tube with a stop ring formed thereon and an ambient pressure side. The blow out preventer is connected to the entrance of the subsea hydrocarbon producing wellbore, and includes an inflatable bladder and is connected to a hollow riser connected thereto.
The method steps involve first inserting a data transmitting wireline coaxially through a sea floor pressure head assembly, then connecting the wireline to a wireline tool. The wireline tool and the sea floor pressure head assembly is then inserted into the riser attached to the blow out preventer and then the sea floor pressure head assembly is lowered into the blow out preventer until the inflatable bladder circumferentially surrounds the elongated tube along a discrete axial distance. The discrete axial distance is located between the stop ring and the ambient pressure side. Inserting the sea floor pressure head assembly into the blow out preventer produces an annulus that is situated between the blow out preventer and the elongated tube.
Next, the inflatable bladder is inflated to produce a seal where the then inflated bladder circumferentially engages the elongated tube along the discrete axial distance which in turn attaches the elongated tube to the blow out preventer. The inflated bladder occupies the portion of the annulus between the elongated tube and the inflatable bladder which provides a pressure seal in the portion of the annulus between the elongated tube and the inflatable bladder.
A pressurized completion fluid is introduced into the hydrocarbon producing wellbore to increase the pressure within the wellbore. This simulates the pressure experienced by the wellbore when the wellbore is filled with a high density drilling fluid. The wireline tool is then traversed through the hydrocarbon producing wellbore for collecting data from within the wellbore.